The present invention is in the field of thickness measuring and monitoring equipment and is more specifically directed to means for continuously monitoring the thickness of corrugated paper sheets or the like to provide an indication of any undesired departure from a desired thickness range.
Corrugated sheets formed in conventional corrugating machines are relatively hot in that they are at a temperature in the order of 250 degrees F. Such sheets are also abrasive in nature. It is consequently necessary that thickness monitoring means for such sheets be constructed in a manner to be unaffected by the relatively high temperatures and not to have any sliding contact with the sheets due to their abrasive nature. Prior sheet monitoring devices have for the most part been fixedly positioned over the conveyor means by means of which the sheets from the corrugating machine are conveyed with a substantial disadvantage of such equipment being that it merely measures the sheet thickness along a linear line extending along the length of the sheet and consequently fails to detect any dimensional irregularities transversely spaced from such linear line along which the measurements are taken. Consequently, dimensional errors can go undetected while literally thousands of sheets pass the thickness monitoring devices of this type. Other prior art devices have employed radioactive emitters and sensors which while probably safe, nevertheless create some apprehension among users of such equipment. Other problems found in the prior art machines include lack of accuracy, high expense and unreliability.
Prior known thickness monitoring devices are exemplified by the following U.S. patents:
______________________________________ Patentee Patent No. ______________________________________ Shoupp et al 2,264,725 Bendtsen 2,806,373 Mamas 3,006,225 Ziffer 3,084,314 Doodhall 3,182,587 Lefkowitz 3,262,054 Kebelitz et al 3,263,460 Davis 3,306,103 Gerhard et al 3,307,215 Taylor, Jr. 3,403,447 Mangan 3,474,668 Lloyd 3,509,815 Eakman 3,599,288 McCarty 3,600,747 Hays et al 3,691,940 Wennerberg 3,828,248 Wilhelm, Jr., et al 3,914,585 Atkinson 3,974,248 Edwards et al 3,979,935 Sanders 4,031,752 Stanheli 4,041,770 Mitzel 4,068,385 Herzhoff 4,095,063 ______________________________________
Therefore, it is the primary object of the present invention to provide a new and improved apparatus and method for monitoring the thickness of sheet material.
The achievement of the foregoing object is enabled through the preferred embodiment by the provision of first and second linear variable differential transformers mounted on a threaded support shaft above a conveyor so as to have their lower end portions including roller means engagable with the upper surface of the corrugated sheets fed outwardly along the length of the conveyor. Variations in thickness of the corrugated board are detected by the linear variable differential transformers which provides an output signal which is amplified and fed to a attenuator averager. The attenuator averager provides an output signal indicative of the average output signal of the linear variable differential transformer over a predetermined time period which is also indicative of the average thickness of the corrugated board having passed beneath the linear variable transformer during such time period. The output from the attenuator averager is directed to a display which provides a visual readout of the actual thickness of the corrugated cardboard and which is compared by a comparator with a predetermined minimum desired thickness to trigger an alarm in the event that the thickness falls below a predetermined acceptable minimum value. Additionally, the fact that the linear variable differential transformer is being reciprocated transversely with respect to the direction of movement of the corrugated board provides an averaging effect transversely of the board width so that the output of the attenuator averager actually represents the average thickness over a parallelogram shaped area of the corrugated board the exact dimensions of which area are determined by the velocity at which the board is fed beneath the linear variable differential transformers and the velocity with which the linear variable differential transformer is moved transversely across the width of the board.
A better understanding of the manner in which the preferred embodiment achieves the foregoing object will be enabled when the detailed description is considered in conjunction with the appended drawings in which like referenced numerals are used in the different figures for illustration of the same parts.